The invention relates to the calibration of a scanner. More particularly, the invention relates to methods and apparatus for automatically calibrating a scanner. Another embodiment of the invention relates to automatically calibrating a printer.
Many scanning and printing technologies are complex and, due to many different physical and environmental factors, may drift in their color response. For example, in the case of a scanner, the quality of light produced by the light source drifts over time due to the gradual variation of both electronic and optical components of the scanner. In the case of a printer, the amount of color toner or ink that is printed on paper changes over time. It is preferable that printers and scanners have a steady and consistent color response.
Prior art systems attempt to compensate for the difference in color behavior between a measured response of the printer or scanner and an ideal response of the printer or scanner by various calibrating techniques. One common way to calibrate a scanner involves scanning a calibrated target strip with a scanner and then using software that accompanies the calibrated target strip to perform the calibration. The software measures density values on the strip and compares the measured values to stored density values known to correspond to the calibrated target strip. The software generates modified or calibrated density values and typically stores the calibrated values in an association table known in the art as the lookup table (“LUT”). More expensive scanners are capable of running the software, while less expensive scanners run such software on an associated personal computer (“PC”).
Common in the art are two types of calibrated target strips that come with color management software and that can be purchased on the market. The two common strips are the Kodak® Gray Scale calibrated target strip produced by the Eastman Kodak Corp. of Rochester, N.Y. and the ANSI-standard IT8 target manufactured by, for example, the Agfa Division, Bayer Corporation. The Kodak® Gray Scale comprises patches of varying densities of gray and the IT8 strips comprise color patches.
It is known to calibrate a color image reproduction system by generating a scanner calibration table and a printer calibration table. However, the method is elaborate in that it includes the steps of providing a set of calibration tools that are developed on a separate representative system and the steps of applying the calibration tools to calibrate the color image reproduction system. Such a system is described in Wan et al. U.S. Pat. No. 5,452,112.
Dietz et al. U.S. Pat. No. 5,689,110 (“Dietz”) discloses a method and apparatus for calibrating a specialized optical scanner known as a fluorescence spectrometer. One of the apparent significant applications of a fluorescence scanner is in the field of medicine wherein, for example, a blood sample is mixed with a fluorescence dye, and the blood sample is scanned and analyzed to determine certain cell characteristics. The calibration method and apparatus disclosed in Dietz involves such techniques as calibrating to calibration standards, such as gold and rubies, and is not directly relevant to the type of color image scanner addressed in this document.
Quardt et al. U.S. Pat. No. 5,194,966 (“Quardt”) discloses a method for optimizing optical scanners by automatically determining image processing threshold values using a threshold image calibration sheet. One embodiment comprises the steps of repetitively scanning the sheet until scanner threshold values are determined. Another embodiment comprises the step of a single scan but uses two or more scanner cameras to work together to process the document images. The method disclosed in Quardt requires a significant amount of labor to complete repetitive scans and/or requires a scanning device having two or more cameras.
Sherman et al. U.S. Pat. No. 5,537,516 (“Sherman”) discloses a method for calibrating a subject scanner that uses a standard target with respect to a calibrated reference scanner, known as the object scanner. The method generates scanner correction tables for the readings of the subject scanner. The output of the object scanner when scanning a reference target is compared to the output of the subject scanner when scanning the reference target. This method requires a deliberate action on the part of a user whenever calibration is desired in which the user is required to have the target on hand and the user is required to scan the target. Sherman further provides an improved method of measuring ink densities using a subject scanner. In one embodiment, a target is placed at a defocused point with respect to a focal point of the scanning device, and in an alternate embodiment, a diffusing substrate rather than a defocusing means is placed between a target and the scanning device.
It is also known to generate a scanner profile by scanning a standard gray scale test strip comprising a plurality of gray scale patches and comparing the scanned test strip data to a stored data file containing density values known to correspond to the test strip. Such a method is described in Falk U.S. Pat. No. 5,760,913 (“Falk”), which discloses the use of a scanner as a densitometer to calibrate a printer. As in some of the previously described documents, the method and system taught by Falk requires the user to have the standard gray scale test strip or its equivalent readily available and requires the user to scan a calibration target and a test strip on the scanner to perform the desired calibration process.
It would be advantageous to provide an automatic scanner calibration method and apparatus. For example, a user who must perform a calibration of a scanner may have misplaced a calibrated target strip. In the prior art, calibration cannot be performed if a target strip is misplaced.
It would also be advantageous for an automatic scanner calibration method and apparatus to provide a means by which a user can calibrate the scanner during a normal scan of an object. An automatic scanner calibration method and apparatus providing the means to calibrate the scanner during a normal scan of an object has the potential to be more accurate than older in time calibrations and more accurate than a scanner set to the preset default settings provided by the manufacturer. For example, it is well known in the art that a scanner can have a warm up time of about twenty minutes before reaching a steady output value. A user may desire to scan an image prior to a scanner reaching the steady output value. It would be advantageous for the user to calibrate the scanner automatically.